The present invention is directed to a method and apparatus for compression molding plastic articles such as closures, and more particularly to an improvement in the method and apparatus disclosed in U.S. Pat. No. 5,554,327.
U.S. Pat. No. 5,554,327, assigned to the assignee hereof, discloses a method and apparatus for compression molding plastic articles such as plastic closures. A plurality of tools are mounted in a circumferential array on a rotatable turret in a plurality of opposed co-acting pairs. The tools of each pair carry opposed male and female mold sections that together, when closed, form a cavity mold for compression molding the desired articles. The turret is mounted on a machine frame that also carries cams for operative engagement with the tools for moving the tools of each pair toward each other during a portion of motion of the tools around the turret axis for compression molding articles between the tool pairs, and for moving the tool pairs away from each other during another portion of tooling travel for releasing articles molded between the tools. Fluid cylinders are associated with each tool pair to provide a constant limiting molding force for each tool pair. U.S. Pat. No. 5,603,964, also assigned to the assignee hereof, discloses an apparatus for cutting and delivering extruded plastic gobs to the successive molds of the molding apparatus.
Although the methods and apparatuses disclosed in the noted patents address and overcome problems theretofore extant in the art, further improvements remain desirable. For example, it is desirable to hold the tools and mold halves in the closed and compressed position for as long as possible to improve cooling efficiency and part definition and quality during the molding process. However, it is also undesirable to employ the cams on the machine frame for holding the molds closed at normal molding force during most of the operating cycle because this would place undesirable forces on the machine frame and increase cam wear. For this reason, it is conventional practice to reduce mold forces during the curing portion of the cycle, with corresponding reduction in cooling efficiency and part quality. It is also desirable to improve machine efficiency by increasing the tooling density within a machine, and thereby increasing overall productivity per unit of plant floor space. However, employing conventional technology, this would only exacerbate the mold force problem, which in turn would require redesign of the machine frame, cams and bearings.
It is therefore a general object of the present invention to provide a method and apparatus for compression molding plastic articles that permit maintenance of full compression force on the mold elements during the forming and curing of the molded component while reducing wear on the cams that close the mold elements, and while reducing mold clamping reaction forces applied to the machine frame. Another object of the present invention is to provide a method and apparatus of the described character that achieve improved efficiency in terms of tooling density and utilization of plant floor space.
A method and apparatus for compression molding plastic articles such as closures in accordance with the present invention include a plurality of tools mounted in opposed pairs with the tools of each pair including opposed male and female mold sections. The tool pairs are mounted for motion in an endless path. The tool pairs and associated mold sections are closed during motion in the path for compression molding a charge in the mold cavity formed between the mold sections of each pair, and opening the mold cavity to release an article formed between the mold sections. In accordance with one aspect of the present invention, a mechanism is associated with each pair of tools for locking the tools and mold sections in the closed position during motion in the path independent of the mechanism for closing the tools and mold halves. In this way, forces imparted to the mold halves and tools to hold the molds closed are isolated from the machine frame. The locking mechanism is selectively unlocked to permit opening of the mold halves and release of the articles compression-molded between the mold halves.
The opposed tool pairs are mounted in the preferred embodiment of the invention on a slide mechanism that interconnects the tools of each pair. The locking mechanism is disposed to engage the slide mechanism to prevent motion of the tools of each pair away from each other. The slide mechanism in the preferred embodiment of the invention includes a slide shaft coupled to one of the tools of each pair, with the other tool of each pair being slidable on the shaft. The locking mechanism includes a stop on the shaft and a latch disposed on the other tool of each pair for movement between a first position in engagement with the associated stop for holding the tools and associated mold sections in closed position, and a second position in non-engagement with the stop for permitting opening of the tools and associated mold sections. The latch in the preferred embodiment of the invention is urged to the first or latching position by a spring disposed between the latch and the tool on which the latch is mounted, and is moved to the second or non-latching position by means of a latch actuator that extends from each latch for engagement with a cam disposed in fixed position on the machine frame adjacent to the path of tool travel. The latch and stop are so contoured with respect to each other that an increase in molding force within a compression mold cavity increases the force of abutting engagement between the latch and the stop.
In accordance with another aspect of the present invention, each of the tool assemblies carries mold cavity cores and holders for forming a plurality of mold cavities at each tool pair. The mold cavities are equally spaced from each other circumferentially of the mold path, both within each tool set and between tool sets. Density of production is thus greatly increased as compared with prior art in which each tool pair forms only a single cavity. The mold elements and cavities preferably are disposed in arrays on each tool pair concentric with the turret axis. Preferably, each tooling within each array is equally circumferentially spaced from its adjacent tooling within the same array. The mold cavities may be radially aligned or radially staggered with the same number of cavities in each array, or may be disposed such that there is a greater number of cavities in the outer array than in the inner array.